Why use a ground when the nuetral can do the same thing?

They shouldn't be, but often are.

If you short the hot to the box, wouldn't it be nice if the fault current went somewhere other than through you?

If your neutral breaks do you want all your boxes to be at line voltage?

Not in fault conditions they don't.

You don't have an extra "ground" wire. You have a current carrying "neutral" wire and a non-current carrying "ground" wire.

and what happens should your combined neutral and protective ground wire break?

Fishbulb

Why couldnt I just take a jumper from the neutral and ground the box with it? They go to the same place.

The neutral or white wire is a current conducting wire. The ground wire is a safety device/wire. Like safety chains on a towed trailer. Bob AZ

the neutral is a grounded conductor. it is established at the service. It need to remain separate at all other locations.

Yes unless the box is non metallic. If there is no return path then the breakers will not trip

What your describing is the way electricity was installed before circuit breakers. Years ago there was no ground pulled with the conductors to the location of use. People died hence we got smarter and started requiring grounds.

If it is grounded properly then you will not die if there is a fault or short circuit.

Get it right - wire is an electrical component!

The same thing that would happen if my ground wire became seperated from the box.....

I am not saying to not ground the box. I am saying if I put a jumper from my neutral wire to the box it would accomplish the same thing.

If the hot contacted the box the breaker would trip.

I know the way it is 'supposed' to be done .....I just dont know a 'technical' answer for why.

I am looking for a more detailed reason.....just trying to 'really' understand the concept here.

Thanks.

There is another problem I haven't seen addressed although the "open neutral" is the worst case. You are going to have some voltage drop on that neutral as soon as you start drawing current and that will actually present itself as a rise in the voltage on the case of your tool. The higher the resistance of your neutral connections are and the length of the conductors will only make it worse. Out at the end of a long run of 14 ga romex you might start to get a good bite if you were sitting on the garage or basement floor. The objective of the isolated grounding conductor is to give you a low resistance path straight from the case of your hand tool to the grounding electrode with no other use of that wire but to handle fault current. It's like asking why you have safety chains on your trailer since it is already connected to the ball.

No. In the former instance, a grounded appliance is left with a live case. In the latter instance, the case has lost its earthing, but is not live (unless a second fault also exists).

Although they connect together and to the grounding electrode at the service entrance, they perform different functions.

First, we have the "grounded conductor" (neutral), which normally carries current. You do not use that to ground the outside of enclosures, conduit, etc, because you don't want current flow through the surfaces that peo[ple can contact. As was pointed out, if the neutral opened up somewhere in the system, everything on the side of the break away from the service panel would rise to line voltage with respect to earth. Even a high resistance can result in lethal potentials.

Next, we have the "grounding conductor" (green wire) that is used to provide the equipment ground throughout the system. It does not normally carry current, other than small leakage currents from the connected equipment. Therefore, an open equipment grounding conductor would have no negative effect, unless an additional fault occured.

The concept is to not mix the current carrying and safety grounding functions on the same conductors.

Ben Miller

In theory you are partially correct. Using a jumper, and "grounding" the box to the Neutral in the box, in most cases, would cause the breaker to trip if the hot touched the box. There are four reasons I'm aware of for not doing the above.

1) Most current electrical codes I'm aware of prohibit it.

2) Connecting the Neutral to the metallic frame of a piece of equipment, or other metal surfaces, allows the metal surfaces to rise above ground potential where high current flow in the Neutral results in elevated impedance of the Neutral Conductor.

3) Having a very low impedance path on the ground wire, not normally a current carrying conductor, usually results in quicker tripping of the over current device in the event of a short to ground.

4) Electronic devices plugged into a properly grounded circuit are less likely to experience difficulties from an elevated ground reference, and more immune to common mode noise.

I'm sure others here can offer additional reasons why it would ill advised.

Louis--

********************************************* Remove the two fish in address to respond

IOW box should be connected to the far end of the white neutral wire which is not electrically same as load end. Each end of white neutral wire is electrically different which is why Louis Bybee's 1) is a requirement and why 2) 3) and 4) are electrically correct.

Another way of explaining it is called single point grounding. Box and load (neutral) don't meet until they arrive at the single point ground in breaker box. This avoids what could be deadly 'ground loops' that would also involve another electrical connection - earth, pipes, etc.

As I noted > In theory you are partially correct. Using a jumper, and "grounding"

In most boxes, the Ground Bus and the Neutral Bus are already tied together.

But you ground the box because your transformer ground goes to earth, and as ground fault current wants to get back to source, the most direct path is via the two grond rods through earth. And also, usually the grond fault current which is available is far in excess of the rating of your neutral conductor, and the last thing you want to do is melt your life-line. Because you are then literally up the proverbial creek.

HR.

What everyone else has said so far, I agree with.

ANOTHER thing to consider is: Grounding via the neutral means a single failure (open neutral) can create a hazardous condition (voltage between box and consumer). With a separate 'grounding conductor' it would take two failures (open grounding conductor *and* hot wire touching the box).

Two failures to create a hazardous condition is less likely to happen than one. Not 'perfect', but much better.

daestrom

You know, I've read this from many folks and just have a hard time with it. I think one of the main reasons to connect the 'grounding conductor' and the 'grounded conductor' at the service entrance is so current does *not* have to flow through the grounding rods to trip the breaker.

A fault in equipment connecting 'hot' to the casing will allow current to flow back through the grounding conductor to the service entrance. There, it is connected *directly* to the neutral and flows through the neutral back to the pole transformer. The resistance of the grounding rods is not a factor in tripping the breaker.

I'm not saying a good earth grounding system isn't important, just that it isn't what trips a breaker when there is a fault from 'hot' to the 'grounding conductor'. Certainly the tie in the service panel and the neutral conductor from the service panel back to the pole transformer is

*much* lower resistance than150 feet of even wet mud (much less dry sand).

daestrom

Simple really.

1. If you don't connect exposed metalwork to anything, then leakage currents will flow to it, and potentially through you to ground. So if an insulator breaks down, or leaks badly, it can kill you.

1. If you connect exposed metalwork to neutral then it won't be at ground potential, it will be a couple of volts up because of the voltage drop in the neutral. This won't kill you but it will cause corrosion if wet and it could under some circumstances cause fire, it can certainly light a torch bulb because a low resistance connection from neutral to ground will carry the full circuit current albeit at low voltage. What can kill you is the current surge when a fuse or breaker trips. You get a momentary very large current before the trip, in which the whole supply voltage to be dropped half each in live and neutral leads. This short pulse can stop hearts, rarely, although with modern overcurrent protection the risk is not as serious as it once was.

2. Finally, when the breaker does go out there is an inductive pulse, which is what makes it arc. How high and how long depends on lots of things, but that can jump fuses, jump breakers, raise your equipment up to kilovolts for milliseconds and generally create havoc. It also rings at high frequency so can be coupled capacitatively. This pulse is probably more likely to be responsible for deaths. The worst case is probably when there is no load on the supply transformer, a colleague of mine put accidentally put an ohmmeter across a 240V 15A spur under these conditions. His test meter exploded despite all the fuses and breakers in the circuit!

If instead you return the exposed metalwork through a non-current-carrying conductor right back to the electromotive source, usually a transformer, to a point which is electrically connected to the groundwater, then you only have the leakage current to create voltage drop, not the fault current, and all the mayhem stays safely inside its Faraday cage, and you can't get killed by conduction from exposed metalwork through you to the groundwater and back to the source.

Tim Jackson

Most of the fault current will return to the transformer through the copper grounded service conductor, which is in parallel with the path through the earth and has perhaps 50 to 100 times lower resistance!

The main reasons for grounding have to do with stabilizing the system voltages, especially in cases of lightning or contact with higher voltage conductors, and allowing protective devices in the distribution system to function.

Ben Miller